In systems where a variable gain amplifier is needed, there are numerous situations wherein the gain of the variable gain amplifier (VGA) needs to be varied in a logarithmic manner. By varying the gain in a logarithmic manner, the VGA will provide a fairly constant or near linear gain per volt. A linear gain per volt is sometimes called a log linear gain. The inventor notes that known previous VGA circuit designs generally do not provide a smooth log linear curve, but instead provides a curve with bumps or ripples that deviate significantly from a constant slope. The bumps and ripples found in prior circuit designs result in consequences in the overall performance of such prior VGA designs. For example, if one is attempting to amplify an exponentially decaying signal with a smooth log linear amplification curve, then minimal distortion of the amplified signal will result. Conversely, if one uses an approximated log linear curve having bumps and ripples, signal distortion will result at the output of the amplifier.
Another problem produced by bumps and ripples in a log linear curve is that any noise introduced by one or more control lines will modulate the signal in an undesirable manner. The control line noise is directly related to the instantaneous slope of the VGA amplification curve. For example, if you have a gain curve where there is a high slope and a low slope and then a high slope again, the high slope region will exaggerate the control voltage noise at the output of the amplifier.
What is needed is a VGA device that can provide a repeatable, linear gain curve in terms of log linear gain. Furthermore, it would be advantageous to repeatedly provide such a log linear gain curve from a VGA device that does not vary significantly within a predetermined temperature range or within other predetermined environmental variables.
Prior VGA device designs have been found to have one or more disadvantages. Such disadvantages may include having bumps and ripples in the db linear gain, being temperature sensitive over a given temperature range, not being able to provide a repeatable gain on a constant basis or over time or not providing a repeatable output due to the manufacturing process of the device. As such, what is needed is a db linear process independent variable gain amplifier that provides a substantially linear db/volt gain within a given wide operating temperature range, that is insensitive to process variations, and provides a repeatable output or outcome when incorporated into a plurality of devices.